This invention relates to a sodium-sulfur battery and power storage system of large capacity, using the sodium-sulfur batteries.
Sodium-sulfur batteries have been studied and developed in many countries since the principle of the sodium-sulfur battery was disclosed in 1969 [Denki Gakkai Gijutsu Hokoku (Part II) No. 103, 29-36 (October, 1980)].
In the basic structure of a sodium-sulfur battery, molten sodium is used as an active material for a minus pole, molten sulfur as an active material for a plus pole, and sodium ion-permeating solid electrolyte as an electrolyte. The solid electrolyte is a glass or a ceramic, and particularly .beta.-alumina (Na.sub.2 O.ll Al.sub.2 O.sub.3) is used as the electrolyte in most of the batteries now under development owing to high sodium ion permeability. Furthermore, .beta.-alumina also acts as a separator between the plus pole and the minus pole because it has no electron conductivity.
Sulfur also has no electron conductivity, and thus is impregnated in an electroconductive material to transfer electrons accompanying the electrochemical reactions. Effective working temperature for the battery is 300.degree.-350.degree. C. in view of the melting point of the active material for the plus pole. One example of typical structure of the battery is disclosed in Japanese Patent Application Kokai (Laid-open) No. 57-18670.
Discharging and charging reactions of the battery can be given in the following equations:
At a minus pole: ##STR1##
At a plus pole: ##STR2##
Throughout the battery: ##STR3##
The sodium-sulfur battery has the following characteristics owing to the electrolyte being a solid and the active materials for both poles being molten liquids:
(1) No side reaction occurs at both charging and discharging, resulting in no self-discharge, and charging quantity of electricity can be equal to the discharging quantity of electricity.
(2) The battery is of a completely sealing type, resulting in no maintenance requirement.
(3) The active materials are liquids, resulting in no life limit to the battery.
(4) Capacity reduction is much less at a high rate discharging.
However, the battery has the following problems:
(1) When the solid electrolyte is damaged owing to a mechanical shock, thermal shock, etc., or cracked through attack by potassium as an impurity in sodium, there is a danger of rapid reaction between sodium and sulfur, and there are no measures for remedying the danger.
(2) Molten sulfur has a high activity and is also used at a high temperature, and corrodes a metallic vessel.
(3) Battery characteristics are deteriorated due to a decrease in purity of molten sodium and sulfur, resulting in shortened life of the battery.